1. Field of the Invention
The present invention relates to a switching power supply device which stabilizes an output voltage using a digital control circuit.
2. Description of the Related Art
In a related art switching power supply device, an output voltage is stabilized by, for example, pulse width modulation (PWM) control or pulse frequency modulation (PFM) control using a highly intelligent digital control circuit with, for example, a digital processor. Especially recently, it is urgently required to reduce a transient fluctuation in an output voltage in response to a sudden change in an external environment of the power supply device.
Exemplary situations of sudden changes in an external environment of the switching power supply device may include a sudden change in an input voltage during operation or a sudden increase or decrease in an output current in response to a change in a state of a load. A starting operation of the switching power supply device when an input voltage is placed in the switching power supply device which is not in operation also becomes a problem.
For example, a switching power supply device used in information and communication equipment or a server needs to have high speed response characteristics of output voltage control in response to a sudden change in the load because a situation in which an output current suddenly changes from about zero to rated load may occur frequently. Further, in a system in which a plurality of board substrates are supplied with voltage from a single input power line, it is assumed that, when another board substrate is hot swapped during maintenance, a loading state of the power source line of the system changes and an input voltage of the switching power supply device which receives power supply from the power source line changes suddenly. Therefore, the switching power supply device used in this kind of system needs to have high speed response characteristics of output voltage control in response to a sudden change in the input. As a general matter, if overshoot or vibration occur when an input voltage is placed and an output voltage increases from zero to reach a predetermined output voltage, there is a possibility that electronic equipment as a load may malfunction or break down. Therefore, a switching power supply device needs to start with an output voltage thereof increasing monotonously.
In order to solve these problems, as disclosed in, for example, PTL 1, a switching power supply device which obtains a signal corresponding to an input voltage during operation (input voltage signal), a signal corresponding to an output voltage (output voltage signal) and a signal corresponding to an output voltage differential value which represents a change in the output voltage (output differential signal), performs a predetermined digital calculation process on the basis of each signal, and determines ON time and OFF time of a main switching element is proposed. In this switching power supply device, a control function formula which is a negatively sloped linear function and which prescribes a relationship between an output voltage signal and an output differential value is defined in a calculation means of a control circuit. The calculation means samples the input voltage signal, the output voltage signal and the output differential signal at a timing synchronized with the switching cycle of the main switching element, and calculates the ON time and OFF time of the main switching elements thereafter so as to satisfy the control function formula described above. The drive pulse generating means generates drive pulses which turn the main switching element ON and OFF, on the basis of the ON and OFF time determined by the calculation means. At this time, the output differential signal is obtained by a capacitor current detection means which measures a current of a smoothing capacitor, or a differential means which differentiates the output voltage signal.